Custom error messages with validation message tag helper

[coxp]

Is this a Bug or Feature request?:

Feature request or documentation request

Steps to reproduce (preferably a link to a GitHub repo with a repro project):

Add a simple model and a view that uses the validation message tag helper.

public class Entity
{
    [Required]
    public string Name { get; set; }
}

<span asp-validation-for="Name">Custom error message</span>

Validation message is displayed as "The Name field is required" rather than "Custom error message"

Description of the problem:

Simple workaround is to add the data-valmsg-replace="false" along with the asp-validation-for attribute.

<span asp-validation-for="Name" data-valmsg-replace="false">Custom error message</span>

The ValidationMessageTagHelper does check for inner content but the data-valmsg-replace attribute has already been set to true at this point as a null message parameter is passed to the HtmlGenerator. Ideally the data-valmsg-replace would be set to false at this point if the element has inner content. Alternatively change the validation message documentation to show how to set the data-valmsg-replace attribute for custom error messages.

Version of Microsoft.AspNetCore.Mvc or Microsoft.AspNetCore.App or Microsoft.AspNetCore.All:

2.1.1

[mkArtakMSFT]

Thanks for contacting us, @coxp.
@dougbu, looks like this is a pure documentation related. Though I'm not sure we don't have this documented properly. If not, let's use this issue to fix/add documentation.

[dougbu]

@mkArtakMSFT since the current behaviour is counter-intuitive, suggest we update ValidationMessageTagHelper to detect existing content before calling into the IHtmlGenerator. That is, we should treat overwriting the explicit <span> content as a bug.

[mkArtakMSFT]

Sounds good, @dougbu.
To avoid breaking existing users, we should use a compatibility switch to opt into the proposed behavior.

[dougbu]

Thanks to @kishanAnem for PR #8087. He's well on the way to fixing this issue.

[kishanAnem]

@dougbu welcome. :) . I'm glad to work with you.

[dougbu]

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